The chalcopyrite thin film solar cell can be manufactured by a variety of manufacturing processes from a wide range of materials, and is therefore susceptible of low cost manufacture and improvement in performance. For this reason, solar cells of this type have been the subject of intensified research and development efforts by a number of research institutions and manufacturing corporations. In recent years, commercialization of chalcopyrite thin film solar cells are being accelerated because of the ease in fabricating large area devices and the low energy consumption in the fabrication process. The chalcopyrite materials preferred for such commercialization include Cu(In, Ga)Se2 and Cu(In, Ga)(SeS)2.
A typical chalcopyrite thin film solar cell has a layered structure, and includes a substrate, a back electrode layer, an absorber layer (a light absorber layer), a buffer layer and a transparent electrode layer that are stacked one above another in that order from the bottom layer or the substrate. The absorber layer has the function to absorb light energy and convert it into electric power, and is therefore considered to be crucial in maximizing the generation and capture of carries in the absorber layer, and improving the performance of the solar cell. It was proposed to progressively increase the band gap of the semiconductor material of the absorber layer from the side of the buffer layer to the side of the back electrode layer. See PCT pamphlet WO2004/090995 (patent document 1). This patent document also discloses a method for forming an absorber layer which includes the steps of forming a Ga layer on a back electrode layer by sputtering or CVS, depositing a Cu(In, Ga)Se2 layer on the Ga layer and thermally diffusing Ga atoms into the Cu(In, Ga)Se2 layer. Changing the ratio of In to Ga causes the band gap to change in a corresponding manner.
Patent document 1 discloses a method for forming an absorber layer in which the band gap is varied by changing the ratio of the composition elements along the direction of the thickness of the film (band gap gradient structure). As this method controls the band gap gradient structure by a thermal diffusion of Ga in an intermediate step, the gradient is dictated by the diffusion constant of Ga, and cannot be controlled at will. When a molecular beam epitaxy (MBE) film forming process, which may be considered as a vapor deposition process, is known to allow the distribution of the ratio of the composition elements to be varied in a progressive manner or the band gap gradient structure along the thickness of the film to be controlled in a relatively precise manner. However, the MBE process is considered to be unsuitable for mass producing solar cells as it is required to be performed in a high vacuum environment, and is not only expensive and time consuming to perform but also unsuitable for processing large surface area devices. There may be other vapor deposition processes and sputtering processes that may be considered as being suitable for mass production, but they do not allow a precise control of composition because of various reasons such as the limitations in the composition of the sputter targets. Therefore, these processes are unsuitable for providing a desired band gap gradient structure.